Seat occupancy by motor vehicle passengers plays a major role in a plurality of technical applications in motor vehicles. This applies especially to vehicle occupant restraint systems, the efficient deployment of which is very often made dependent upon the seating position of the vehicle occupant.
The simplest form of recognizing seat occupancy is the detection of the presence of a vehicle occupant by, for example, touch-sensitive switches in the vehicle seat. Other measurement systems for the recognition of seat occupancy detect the weight of a vehicle occupant or even their weight distribution on a vehicle seat. On the one hand, this enables the detection of the presence of a vehicle occupant whilst, on the other hand, their body weight or weight distribution on the vehicle seat can be determined. Depending on the sensor arrangement and measurement method, these two sets of data can be measured simultaneously or independently of each other.
Seat mat sensors are mostly used to detect the measured variable, in particular weight, comprising a plurality of pressure-sensitive sensor elements which are arranged on the seat face distributed in rows and columns.
Known from the prior art are, in particular, sensor elements in seat mat sensors essentially comprising two electrically separating films arranged in a mutually parallel manner. Between said two films is disposed a likewise electrically separating interface layer which maintains the distance between the two films. Electrically conductive faces are applied to the mutually facing sides of the films between which high-resistance material is arranged, preferably air.
When the seat mat sensor is subject to pressure, for example the body weight of a vehicle occupant, the conductive faces are brought into conductive contact with each other, so that the electrical contact resistance between the two conductive faces is dependent on the pressure amplitude.
The conductive faces of known sensor elements, hereinafter also referred to as sensor cells, have rotational symmetry about the face perpendicular. In a sensor arrangement in the form of a seat mat sensor, a plurality of similar circular sensor cells are arranged in a planar manner on a vehicle seat.
Sensor arrangements of this kind are known from U.S. Pat. No. 5,896,090 A, German Application No. 200 14 200 U1 and German Application No. 42 37 072 C1; suitable sensor cells for these sensor arrangements are disclosed in U.S. Pat. No. 4,314,228, German Application No. 200 14 200 U1 and U.S. Pat. No. 4,314,227.
The problem with the known sensor arrangements is that the sensor cells are mostly affixed to an uneven surface of a vehicle seat thereby exposing the sensor cells to differing tensile or bending loads, depending on their position on the seat mat sensor, in particular along fold lines of the sensor arrangement.
The sensor elements known from the prior art are designed to respond very sensitively to such tensile or bending loads on the fold lines. Consequently the signal characteristic of the sensor elements dependent on the weight acting upon them changes. The bending of the sensor elements usually displaces a constant offset PL of the signal characteristic to lower values as shown in FIG. 4. This signal characteristic displacement can have various repercussions for various measurement systems designed for recognizing seat occupancy.
An occupant may be detected on the vehicle seat even if only a small, light object is located on it. In the event of vehicle impact this false information would cause an airbag to inflate unnecessarily in order to protect the supposed vehicle occupant.
In other systems designed for recognizing seat occupancy, a shift in the constant offset of the sensor characteristic causes a false weight or false weight distribution of a vehicle occupant to be reported to the occupant safety system. If an occupant safety device is, for example, released too early on the basis of this false information, this can result in serious injuries to a vehicle occupant.
The change in the sensor characteristics when the sensor element is exposed to a tensile or bending load on a vehicle seat fold line is referred to as the preload effect.
Owing to the preload effect an arrangement of sensor cells in seat mat sensors is often avoided at heavily curved points or on seams of a vehicle seat, although the lack of measurement at such points results in a loss of important information regarding the vehicle occupant. This can also lead to serious injuries to a vehicle occupant in the event of an accident involving impact if the occupant safety system was unable to provide the optimum protection for the vehicle occupant through lack of information.
A further measure for avoiding the preload effect is to reduce the size of the conductive faces of the known sensor cells.
With small sensor cells the constant offset (PS) of the sensor characteristic generally has a higher value than with larger sensor cells (FIG. 4). As a result, the preload effect manifests itself only at far greater bending loads than in the case of large-face sensor elements. However, the sensor characteristic of a smaller sensor cell (CS) loses some of its resolution accuracy by comparison with the characteristic of a larger sensor cell (CL), since the slope of its signal characteristic becomes steeper. At the same time the range of values of its characteristic also becomes smaller (FIG. 4).
Here again as a result of too much imprecise information regarding the vehicle occupant, an optimal protection effect of an occupant protection system adapted to seating occupancy fails to take place.